I think it’s at least a couple things.
State and other types of systemic violence (unless it’s an actual war) aren’t really vivid events. They’re just always there in the background, so people become desensitised to it, and the violence itself becomes somewhat normalised and even given rationalisations (and often flatly denied).
States are vastly more powerful entities and can employ a lot more, and more effective propaganda. Many of their citizens will internalise and willingly spread state propaganda for free, justifying, rationalising, underplaying and outright denying the state violence.
Terrorist attacks are extremely vivid and attention grabbing events that provoke strong emotions and outrage in the way systemic state violence doesn’t, even if the victims of the latter are greater.
The media will give relatively a lot more attention to crimes of official enemies, whether state or terrorist groups. Our own state crimes, official allies, and states deemed geopolitically important or expedient, tend to be downplayed, rationalised, or treated as mere regrettable mistakes rather than immoral crimes.
“Are there legitimate targets that terrorism might be used upon? ”
There’s a heavier burden of justification, and I think it’s easier to imagine targeted assassinations being justifiable in some contexts, regardless of overall efficacy (maybe some bastard just deserves to die), while indiscriminate bombing or attacks on civilians are pretty much unjustifiable.
“Are there legitimate uses of apartheid?”
No.